In recent years, a demand for high-strengthening of steel sheets used for automobiles and the like has been increasing, and high-strength steel sheets having a maximum tensile stress of 900 MPa or more are also being used.
These high-strength steel sheets are formed in large quantities and in an inexpensive manner through presswork similar to mild steel sheets, and are provided as members. However, in accordance with a rapid acceleration of high-strengthening in recent years, there has been a problem that in a high-strength steel sheet having a maximum tensile stress of 900 MPa or more, a springback is caused right after press forming, and it is difficult to form a target shape.
As a technique of improving a shape fixability of a conventional high-strength steel sheet, there can be cited a hot-dip galvanized steel sheet with high strength and high ductility excellent in shape fixability being a steel sheet containing, in mass %, C: 0.0001 to 0.3%, Al: 0.001 to 4%, Mn: 0.001 to 3%, Mo: 0.001 to 4%, P: 0.0001 to 0.3%, and S: 0.01% or less, having a plating layer containing Al: 0.001 to 0.5%, Mn: 0.001 to 2%, Fe: less than 20%, and a balance composed of Zn and inevitable impurities, containing ferrite or ferrite and bainite of 50 to 97% in total in volume fraction as a main phase, containing austenite of 3 to 50% in total in volume fraction as a second phase, and having a yield ratio of 0.7 or less (refer to Patent Document 1, for example).
Further, as a technique of improving a shape fixability of a conventional high-strength steel sheet, there can be cited a high-strength steel sheet excellent in workability and shape fixability having a structure which contains, in mass %, each of C: 0.06 to 0.6%, Si+Al: 0.5 to 3%, Mn: 0.5 to 3%, P: 0.15% or less (0% is not included), and S: 0.02% or less (including 0%), contains tempered martensite of 15% or more in an area ratio with respect to the entire structure, contains ferrite of 5 to 60% in an area ratio with respect to the entire structure, contains a retained austenite phase of 5% or more in a volume ratio with respect to the entire structure, and may further contain bainite and/or martensite, in which a proportion of retained austenite phase, out of the retained austenite phase, that transforms into martensite by applying a strain of 2% is 20 to 50% (refer to Patent Document 2, for example).
Further, as a technique of improving a shape fixability of a conventional high-strength steel sheet, there can be cited a manufacturing method of a high-strength cold-rolled steel sheet excellent in impact property and shape fixability in which a slab having a composition of C: 0.08 to 0.18 mass %, Si: 1.00 to 2.0 mass %, Mn: 1.5 to 3.0 mass %, P: 0.03 mass % or less, S: 0.005 mass % or less, and T.Al: 0.01 to 0.1 mass %, and having a segregation degree of Mn with respect to a cast slab of 1.05 to 1.10 is hot-rolled, the resultant is further cold-rolled, the resultant is then heated for a retention time of 60 seconds or more in a two-phase region or a single-phase region at 750 to 870° C. in a continuous annealing line, cooling is then performed in a temperature region of 720 to 600° C. at an average cooling rate of 10° C./s or less, cooling is then performed until the temperature reaches 350 to 460° C. at an average cooling rate of 10° C./s or more, retention is performed for 30 seconds to 20 minutes, and cooling is then performed until the temperature reaches a room temperature to obtain a five-phase structure of polygonal ferrite, acicular ferrite, bainite, retained austenite phase, and martensite (refer to Patent Document 3, for example).
Further, as a technique of improving a shape fixability of a conventional high-strength steel sheet, there can be cited a high-strength steel sheet excellent in formability and shape fixability characterized in that it is mainly formed of a ferrite phase of 20 to 97% in volume fraction and a retained austenite phase of 3% or more in volume fraction, in which a proportion of a part other than the ferrite phase having an aspect ratio of crystal grains of 2.5 or less is 50 to 95%, and the steel sheet preferably contains C: 0.05 to 0.30 mass %, Si: 2.0 mass % or less, Mn: 0.8 to 3.0 mass %, P: 0.003 to 0.1 mass %, S: 0.01 mass % or less, Al: 0.01 to 2.50 mass %, and N: 0.007 mass % or less, in which Si and Al satisfy a relation of Si+Al>0.50 mass % (refer to Patent Document 4, for example).
Further, the present applicant discloses a high-strength steel sheet excellent in ductility and stretch flangeability, containing predetermined components, and having a steel sheet structure composed of, in volume fraction, a ferrite phase of 10 to 50%, a tempered martensite phase of 10 to 50%, and a remaining hard phase (refer to Patent document 5, for example).